This invention relates generally to imaging, and more particularly to determining the mass and the volume of soft matter in reconstructed images and, for medical images, suppressing and/or removing tagged material in images. Configurations of the present invention are particularly useful in medical and diagnostic colonography examinations.
In some known CT imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units” (HU), which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two-dimensional slice taken through the object.
To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical, weighting scheme, a method can be derived to multiply the weight with the projection data prior to the filtered backprojection algorithm.
Colon cancer is the third largest cause of cancer-related mortality in the United States, with an estimated 57,000 deaths in 2003. U.S. cancer statistics show that a person has an approximately 5% chance of getting colon cancer in his or her lifetime. Colonic polyps form a pre-cancerous stage of the disease, and if these are removed in time in a person, that person has about a 90% chance of surviving five years or more.
Colonoscopy has been a preferred prior art method for screening for colon cancer. Colonoscopy has an acceptance of 37% as a result of its invasive nature. It has been hoped that CT colonography will increase the acceptance of colon cancer screening by restricting the use of invasive colonoscopy screening procedures only to those that are necessary. However, the reported sensitivity of known CT colonography methods (currently 60-70%) has not yet reached a level comparable to colonoscopy methods.
A common complaint of patients undergoing colonoscopy procedures is the cathartic nature of the colon cleansing prior to the exam. To most patients, this cathartic colon cleansing equates non-invasive CT colonography to the invasive colonoscopy procedure. In response to the discomfort felt by most patients, a non-cathartic or minimum cathartic preparation has been developed that tags liquid and solid fecal mater for subsequent removal using digital subtraction algorithms. These subtraction techniques, however, blindly remove the tagging material or artificially smooth the surface between the removed material and colon lumen. In addition, there is no teaching or suggestion in either of these techniques incorporating the inherent system deformations that happen due to the imaging process.